SILIC10US SPRINGS. 215 



ach side, by which the water escapes wheji the basin is full. It 

 is said that an eruption may be brought on in a few minutes by 

 throwing stones down the pipe, these are again ejected, oftentimes 

 wjth immense violence. The theory of the action of these hot 

 springs of Iceland has not yet been satisfactorily given ; the heat 

 however, is supposed to be derived from subterranean volcanic 

 fires. The silicious water from these springs incrusts plants, 

 twigs, and leaves, similar to the calcareous springs. In the island 

 of St. Michael there are hot springs very strongly impregnated 

 with silica; wherever the water has flowed, sinter or precipitated 

 rock, is formed intermixed with the clay, including grass, ferns, 

 and reeds, in different stages of petrefaction ; branches of the 

 same ferns which now flourish in the island are found completely 

 petrified, preserving the same appearance as when vegetating. 

 There are many springs in this country which deposit silicious 

 and calcareous matter. 



Iron is found in the waters of almost all springs, and some of 

 them are so copiously impregnated with this metal that they stain 

 the rocks or herbage over which they flow. The iron which is 

 thus borne out of the earth and deposited into the sea, acts as a 

 cement to bind together the subaqueous deposits now forming. 



Many of the ancient sandstones are cemented and colored by 

 iron, and pebbles are firmly bound together in ferruginous con- 



